The progressive reduction in feature size in integrated circuits (ICs) is driven in part by advances in lithography. ICs may be created by alternately etching material away from a chip and depositing material on the chip. Each layer of materials etched from the chip may be defined by a lithographic process in which light shines through a mask, exposing a photosensitive material, e.g., a photoresist.
The ability to focus the light used in lithography, and hence to produce increasingly smaller line widths in ICs, may depend on the wavelength of light used. Current techniques may use light having a wavelength of about 193 nm. The use of xe2x80x9csoftxe2x80x9d x-rays (wavelength range of xcex≈10 to 20 nm) in lithography is being explored to achieve smaller desired feature sizes. Soft x-ray radiation may also be referred to as extreme ultraviolet (EUV) radiation.
A challenge in EUV lithography is to eliminate all but a narrow desired range of wavelengths from the light used to expose the wafer (typically 13.4 nm with a 2% bandwidth). The multilayer coatings of the mirrors naturally perform this task in the neighborhood of the exposure wavelength, as their reflectivity rapidly falls to nearly zero away from this wavelength. However, much longer wavelengths may still be reflected by the multilayers. In particular, infra-red (IR) and deep ultraviolet (DUV) wavelengths may still reflect off the mirrors. Such wavelengths are eliminated by use of a Spectral Purity Filter (SPF). These can be made of, for example, a thin-film which transmits EUV and absorbs unwanted wavelengths. Another approach is to use a diffraction grating to spread out the wavelengths, and then spatially block non-EUV wavelengths. Critical performance criteria for SPFs are efficiency (% EUV transmitted) and the ability to cool the SPF (absorbed radiation will heat the element). For example, film-type (transmissive) spectral purity filters may absorb 50% or more of incident EUV radiation.